I just bought a Canon dSLR camera with an 18-135mm IS lens. I really want to take care of it, as I paid extra for this "better" lens, so can the lens get, in some way, damaged if you drop it, say 5 cm, by accident? Is there any test that can be made to see whether or not it was damaged?

2 Answers
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The easiest way to test a lens for damage is to take a photograph using the lens and look for any unusual softness (either all over, or uneven sharpness across the frame), lack of contrast or other striking defects that weren't there before.

You might also notice stiffness or restrictions in the zoom or focus ring operation that can indicate damage, or failure of systems such as autofocus or image stabilisation.

Yes, lenses can be damaged by impact, the likelihood of this occurring depends on:

The height the lens was dropped from. 5cm isn't all that high so the lens wont be going too fast.

The surface dropped onto. A soft surface will slow a falling lens more gradually and be less likely to cause damage.

The build quality of a lens. Some high end metal body lenses are designed to take knocks. I dropped my 135 f/2.0L onto a hard surface from waist height and it was fine, though see the next point.

Luck.

By the same token lenses can be damaged when other objects come into contact with them.

The other common cause of damage to lenses is moisture. Either getting the lens excessively wet during a rainstorm, or storing it in damp conditions which allow mould to develop inside the lens. Again some high end lenses are designed to handle wet conditions.

You can't really use a lens without putting it in harms way, so unless you leave it at home wrapped in cotton wool, it may get damaged. By far the best way to protect your lens is taking out insurance. The latter will also protect against theft, you might even be covered under your household insurance so check the policy.

+1 What unites the first two bullets is that the acceleration causes the damage, not the speed of the lens on impact. E.g., a 2 meter drop onto a trampoline is probably fine, but a 5 cm drop onto concrete might not be. Even worse are local stresses caused by drops onto hard pointy objects, such as a pebble lying on that concrete or having the lens oriented in an unfortunate way: these are the elements of luck mentioned in the fourth bullet.
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whuberJun 14 '11 at 18:25

Dropping the lens accidentally on a ground with stones etc. can do a terrible things. As the lenses are really expensive (unluckily I am not a millionaire), I bought a good carrying camera case. I already had a bad experience with some Vanguard "pistol" case. I accidentally slipped on a greasy road and although I tried to protect the camera, I found that the front lens UV filter is cracked.

So I bought new case (CaseLogic) with more protected bottom (this new plastic shell on the bottom) and also a new UV filter. I use these filters only for protection - it is good - you do not have to care much about fingerprints, very useful also near sea (the salt water is also problem).

As the subject of the question is protection I don't think it violates the guidelines to state that uv filters offer limited protection considering the cost (i.e. they don't protect against hard frontal impacts (the front element does a good job against light impacts by itself), side impacts, water etc.) and can degrade the quality of images in certain circumstances by causing flare/reflections/loss of contrast. Having said that some weather sealed lenses are designed to operate with front mounted filters.
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Matt GrumJun 14 '11 at 16:53

Lens hoods will protect against many of the same types of impacts as UV filters, and will generally improve contrast instead of degrading it. That said, there are cases where a filter can protect your lens where a hood might not, such as when water, sand, or dust is blowing around.
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Evan KrallJun 15 '11 at 6:24

Beware on backpacks and other camera bags. Sometimes you'll leave it open and stuff flies out of it when you pick it up...
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t3mujinJun 15 '11 at 14:15